1. Field of the Invention
The present invention relates to a charging method and a charging device for charging a charge receiving member. In particular, the invention relates to a charging method and a charging device for charging a charge receiving member through a portion in contact with the charge receiving member. More specifically, the invention relates to a charging method and a charging device having a member to be in contact with an electrostatic latent image carrier such as a photosensitive member in an image forming apparatus for charging the electrostatic latent image carrier to a predetermined potential.
2. Description of the Related Art
In electrophotographic image forming apparatuses such as a copying machine and a printer, a charging device charges an electrostatic latent image carrier such as a photosensitive drum, image exposure is effected on a charged area to form an electrostatic latent image, the electrostatic latent image is developed into a visible image, and this visible image is transferred and fixed onto a transfer member. A charging device is employed for charging the surface of the electrostatic latent image carrier prior to formation of the latent image. The apparatus also employs other charging devices such as a transfer charger for transferring the visible image formed on the electrostatic latent image carrier onto a transfer member and a separating charger for separating the transfer member bearing the transferred visible image from the electrostatic latent image carrier.
As the charging devices, corona charging devices such as a Corotron charging device or a Scorotron charging device utilizing corona discharging have been used. Although the corona charging device can advantageously perform stable charging, it requires a high voltage and produces a large amount of ozone. In recent years, therefore, contact charging devices which can replace the corona charging devices have been proposed.
The contact charging device has a charging member which is subjected to a voltage and is brought into contact with a charge receiving member. The charge receiving member is charged by discharge occurring at a gap between a region, which is continuous to a contact region of the charging member with the charge receiving member and is spaced from the charge receiving member, and the surface of the charge receiving member.
These contact charging devices have such an advantage that they produce a particularly smaller amount of ozone than the corona charging device. Also, the contact charging device can operate with a voltage lower than that required by the corona charging device.
The charging member may take various forms such as a charging roller and a charging brush, and other forms such as a sheet-like charging member made of a film have been proposed. In particular, the sheet-like charging member is advantageous in view of reduction in size and cost of the charging device.
However, the contact charging device, and particularly the charging device employing the sheet-like charging member may suffer from the following problem.
The surface of the charge receiving member may be irregularly charged into a scale-like form (scaly form). Therefore, if the charge receiving member is an electrostatic latent image carrier and a mesh image of 1-dot/4-dots is formed as shown in FIG. 5, irregularity in image density occurs in the mesh image due to lack and/or drop of dots forming mesh points. This results in scale-like noises as shown in FIG. 6. This means that the surface of the charge receiving member was not uniformly charged, and charging irregularity occurred in a scale-like form.
Charging of the charge receiving member is primarily performed by discharging occurring at the gap between the region, which is continuous to the contact region of the charging member with the charge receiving member and is spaced from the charge receiving member, and the surface of the charge receiving member. However, charging is also performed by injection charging and specifically by injection of charges from the charging member at an area through which the charging member is in contact with the charge receiving member. This injection charging is strongly affected by a surface resistance of the charging member, and the surface resistance is strongly affected by environmental variation (temperature and humidity). Therefore, the charged potential of the charge receiving member varies with environmental variation as a whole. This prevents stable formation of good images.
Further, a pin-hole may be formed at the surface of the charge receiving member, in which case damages such as scorch and burn may occur at the charging member.
For example, when the contact charging device charges the photosensitive member, foreign matters such as shaved powder of the photosensitive member, residual toner and paper powder gradually stick locally to the surface of the charging member opposed to the photosensitive member as the printing and therefore image formation are repeated and the charging device is operated for a longer term. Abnormal discharging occurs at the portion bearing the foreign matters thus accumulated, so that the surface portion of the photosensitive member opposed to this portion bearing the foreign matters is abnormally charged in a brush-stroke-like form, so that brush-stroke-like noises occur at the image. FIG. 7 shows an example of an image (mesh image) bearing the brush-stroke-like noises.